Semiconductor quantum dots (QDs) with size-tunable band gaps, high photoluminescence (PL) quantum efficiency (QE), and high color purity have shown a great potential for next-generation lighting and displays. Light-emitting devices embodied by QDs have two general forms: First, hybrid QD-organic light-emitting diodes (LEDs), which utilize QDs as the electroluminescent layer while the organic semiconductor layers are responsible for electron-hole injection. Second, light-converting LEDs, in which QDs are excited by wide band gap LEDs and emit the desired PL in longer wavelengths. Recently, using a state-of-the-art white QD-LED backlight system composed of InGaN blue LEDs and multiply-passivated green- and red-light-emitting QDs as light converters, a high performance LCD panel was successfully demonstrated for the first time. However, it is worthy of notice that previous QD-light-emitting research was predominantly based on group II-VI semiconductor QDs, such as CdSe, CdZnSe or CdZnS cores with single or multiple shells. Although such complex and exquisite hetero-structures often lead to outstanding specifications, for instance almost 100% PL QE, however, the high synthesis cost and the toxicity from their heavy-metal ingredients might shadow their potential for large-scale production and wide-spread commercialization.
Group IV silicon QDs (SiQDs), on the other hand, have gradually received more attention, owing to their heavy-metal-free composition, chemical stability and abundant starting materials. Recently, hybrid SiQD-organic LEDs have demonstrated electroluminescence from infra-red (IR) to visible wavelengths. Extensive works have been contributed to the synthesis of SiQDs. To date, main strategies include solution-based precursor reduction, heat-, laser- or plasma-induced aerosol decomposition of SiH4, thermal processing of sol-gel polymers derived from HSiCl3 and harvesting from nano-porous silicon. Except the last one, all the other methods inevitably require critical conditions, special equipment or complex chemical reactions, all of which make them hard to achieve cost-down and scale-up. In contrast, porous silicon can be easily prepared by electrochemical etching in a mixture of common chemicals under ambient condition. The subsequent physical harvesting can effectively separate the SiQDs from the silicon substrate. Noticeably, these highly luminescent powders mostly comprise micro-size silicon pieces with PL-emitting nanocrystal SiQDs trapped on their surfaces, rather than free-standing SiQDs.
Despite recent advances, if silicon-based phosphors are to be used in lighting devices, improvements in the phosphor materials themselves, as well as the methods for making the phosphors, must be achieved.